1. Field of the Invention
The present invention relates to an intermediate transfer type image forming apparatus for transferring plural toner images onto an intermediate transfer medium while superposing the toner images on the intermediate transfer medium, and then collectively transferring these toner images on a transfer medium, and particularly to an improvement of such a type of an image forming apparatus that the transfer of a toner image from an intermediate transfer medium to a transfer medium is carried out in a contact transfer mode.
2. Description of the Related Art
There has been known a conventional intermediate transfer type image forming apparatus equipped with photosensitive drums on which toner images corresponding to electrostatic latent images of respective color components are respectively formed, an intermediate transfer belt (intermediate transfer medium) onto which the toner images on the respective photosensitive drums are successively and intermediately transferred, a primary transfer device for transferring the toner images on the photosensitive drums onto the intermediate transfer belt, and a secondary transfer device for collectively transferring the toner images on the intermediate transfer belt onto a sheet (transfer medium).
Here, for example, a transfer roll or the like which is disposed in contact with the intermediate transfer belt is used as the secondary transfer device. The transfer roll serves to form an electric field between the intermediate transfer belt and the transfer roll to move the toner images on the intermediate transfer belt to a sheet side.
It has been known in some of the above type image forming apparatuses that an image quality adjusting image for adjusting image density, image registration, etc. is formed in an area between images on the intermediate transfer belt (inter-image area) or the like.
In this case, when the image quality adjusting image on the intermediate transfer belt is passed through a facing portion to the transfer roll, the image quality adjusting image would be transferred onto the transfer roll if the electric field for moving a toner image on the intermediate transfer belt to the sheet side is kept to be formed between the transfer roll and the intermediate transfer belt, so that the surface of the transfer roll is soiled by the image quality adjusting image. Therefore, there occurs such a situation that the image quality adjusting image is re-transferred to the back surface of a next sheet to soil the back surface of the sheet when the next sheet is passed.
Accordingly, there has been proposed such a technique that a bias (transfer bias) for forming an electric field directing to move a toner image on the intermediate transfer belt to the sheet side is applied while the sheet is passed through the facing portion to the transfer roll, and a bias (inverse transfer bias) for forming an electric field directing in the opposite direction to the electric field produced by the transfer bias is applied when the sheet is not passed through the facing portion to the transfer roll, thereby preventing the transfer of the toner image in the inter-image portion (image non-transfer area onto the transfer roll (see Japanese Laid-open Patent Application Nos. Hei-8-272235/1996, Hei-8-328401/1996, Hei-9-6146/1997).
Following a recent requirement of further enhancing high image quality, the number of the types of image quality adjusting images to be formed in the inter-image portion on the intermediate transfer belt has been increasing, and the image area required to form an image quality adjusting image has also been increasing. That is, toner amount constituting an image quality adjusting image has been increasing.
Accordingly, it is difficult to perfectly prevent the transfer of the image quality adjusting image onto the transfer roll by merely applying the inverse transfer bias as described above, and thus there occurs such a technical problem that the image quality adjusting image is transferred onto the transfer roll to soil the surface of the transfer roll, and the image quality adjusting image thus transferred is transferred to the back surface of a next sheet to soil the back surface of the next sheet when the next sheet is passed by the transfer roll.
In order to avoid such a problem, there has been known a technique of pressing a blade against the transfer roll to scrape out the toner transferred onto the surface of the transfer roll, thereby avoiding the back surface of the sheet from being soiled.
However, in such a case, the blade must be pressed against the transfer roll under high pressure, resulting in occurrence of such another technical problem that the surface of the transfer roll is worn out and shorten the lifetime thereof, or the blade is tucked up and damaged under a high humidity atmosphere.
As a similar technique has been known such a technique that a conductive brush roll is disposed in contact with a transfer roll, and toner on the transfer roll is electrostatically adsorbed by the conductive brush to thereby avoiding the soil problem of the back surface of the sheet. Here, the conductive brush forms an electrostatic electric field by grounding the brush or applying a bias to it, whereby the toner is electrostatically adsorbed by the conductive brush.
However, in this case, charged toner is kept to be attached to the conductive brush, that is, the toner is trapped in the conductive brush, and thus the toner thus trapped is re-transferred to the transfer roll, so that the surface of the transfer roll is soiled and the toner is re-transferred from the soiled transfer roll to a next sheet to soil the back surface of the next sheet when the next sheet is passed.
In order to remove the toner trapped in the conductive brush, there may be provided a scraping member which abuts against the conductive brush to scrape the toner off the conductive brush. However, since the scraping member is brought into contact with the conductive brush at all times, the conductive brush is worn out, so that the back surface of the sheet is soiled.
In order to solve the above problems, the present invention aims to provide an image forming apparatus which can effectively prevent a next transfer medium from being soiled by an image quality adjusting image transferred to a contact transfer member such as a transfer roll or the like.
The image forming apparatus according to the present invention has an image carrier on which a toner image corresponding to image information is formed, an intermediate transfer medium disposed to face the image carrier, a primary transfer unit that transfers the toner image on the image carrier onto the intermediate transfer medium, and a secondary transfer unit that transfers the toner image on the intermediate transfer medium onto a transfer medium. The secondary transfer unit includes a secondary transfer member having a contact transfer member which is disposed in contact with at least the surface of the intermediate transfer medium, a bias applying unit that applies a transfer bias to transfer the toner image on the intermediate transfer medium through the secondary transfer member to the transfer medium and an inverse transfer bias having the opposite polarity to the transfer bias, and a bias switching unit which switches the bias to be applied by the bias applying unit to apply the transfer bias to an image transfer area on the intermediate transfer medium and apply the inverse transfer bias to at least a part of an image non-transfer area on the intermediate transfer medium. The image forming apparatus further has a disturbing member which is formed of an insulator disposed in contact with the contact transfer member and disturbs the toner adhering to the contact transfer member.